Not sure why an enterprise would want to host their source code at a separate company when it is just as easy to host their own source control system (git, mercurial, tfs, whatever) with their other cloud services (S3, Azure). I do not think enterprises see their source as a "social" connection, and do not see any benefit to thinking of it as such.

1: GitHub Enterprise is an internally hosted solution. They currently distribute it as a VM image.

2: GitHub >>> git

3: Enterprises very much love social in anything they do, they just call it collaboration. See Sharepoint. Also, the pressure from within to use something like GitHub is only going to increase as new hires have experience with it from the outside.